The phrase "a completely different class" is correct and usable in written English.
It can be used to describe something that is entirely distinct or superior in quality or category compared to something else.
Example: "The new smartphone model is a completely different class compared to its predecessor, offering advanced features and better performance."
Alternatives: "a whole new level" or "entirely separate category".
Gleevec was a completely different class of drugs than what was used against cancer.
It will be a completely different class of demand for the shares — when you buy in, you basically get nothing except the hope and prayer of a dividend and some long-term yield.
Of course, the Tesla costs far more and is a completely different class of car (to say nothing of being totally impractical for everyday use until the Model S comes out).
However, in many cases, the symptoms they report are often consistent with a completely different class of diagnoses.
It's not as if BMW is using a completely different class of leather than Ford is using in their Platinum F-150, or that a new Lexus interior has anything on the interior of a new GMC Denali (seriously, have you checked these trucks out? it's ridiculous).
Here is your annual reminder that the rich are a completely different class, earn amounts of money that are unfathomable to the rest of us and that you, yes you, will never ever earn that amount because the system is rigged against you and for them.
"The sensitivity of the information is so extreme and the repercussions of this breach are so extreme, it puts the damages faced by members in a completely different category of class-action suits".
In a single week, I attended three completely different classes.
While first- and second-year undergraduates have usually completed a general education program and decided on a major, transfer students may have taken completely different classes that make the advising process complicated.
Pheromonal synergism between completely different classes of molecules is rare and may be a mechanism to increase combinatorial complexity.
"In the past, scientists thought that these asteroids and comets were completely different classes of bodies.
